In the field of the intelligent transport system (ITS), WAVE (wireless access in vehicular environment) has been proposed as a communication standard for offering various services to an on-board unit (or a vehicle mounting the on-board unit). WAVE, which is a modification of the wireless LAN standard, IEEE 802.11, for a mobile communication, uses IEEE 802.11p in layer 1 (physical layer) and layer 2 (data link layer). One feature of WAVE is that a distributed coordination function (DCF), in which transmission opportunity is equally allocated to each terminal, is performed. In detail, each communication frame is divided into a plurality of time slots each called “channel” in WAVE. Each communication frame includes one control channel and at least one service channel. Broadcasting is performed in a control channel and a communication for transmitting data to a specific destination is performed in a service channel. A source communication terminal desiring data transmission specifies a destination communication terminal and a service channel to be used for the data transmission in a control channel. The destination communication terminal receives data from the source communication terminal in the specified service channel. Multiple services are time-divisionally offered in parallel by alternately repeating communications in the control channels and communications in the service channels. It should be noted that patent literature 1 (Japanese patent application publication No. 2010-239607 A) discloses a technique related to WAVE.
FIG. 1, which is an illustration schematically illustrating one example of communications in accordance with WAVE, illustrates one example of a communication procedure for offering two services A and B to communication terminal #1. In FIG. 1, communication terminal #2 is a communication terminal which offers service A and communication terminal #3 is a communication terminal which offers service B.
Discussed below is an example in which communication terminal #1 transmits data indicating to perform communications for service A and specifying a service channel in which a response is to be done, in a control channel of communication frame #k through broadcasting. Communication terminal #2, which offers service A, transmits data to communication terminal #1 in a service channel in response to the data transmitted by communication terminal #1. Although communication terminal #2 transmits the data in the next service channel, that is, the service channel of communication frame #k in the example of FIG. 1, communication terminal #2 may transmit the data in a service channel other than in the next service channel. Communication terminal #3, with which no communication is requested, does not issue a response even when receiving the data from communication terminal #1 through broadcasting. Further discussed below is the case when communication terminal #1 transmits data indicating to perform communications for service B and specifying a service channel in which a response is to be done, in a control channel of communication frame #k+1 through broadcasting. In this case, communication terminal #3, which offers service B, transmits data to communication terminal #1 in a service channel in response to the data transmitted by communication terminal #1. Multiple services are offered to communication terminal #1 in this way.
One issue is that, in order to achieve switching between multiple services, the respective services are required to use a common frequency for communications in the control channels, whereas the use of the common frequency may cause radio-wave interference due to multipath phasing and overlapping of the communicable regions of antennas, when multiple communication terminals performs broadcasting in a control channel at the same time. The occurrence of radio-wave interference may cause a failure in data communications in a control channel and resultingly cause a failure of switching between services.
FIG. 2 is a diagram illustrating an example of occurrence of radio-wave interference in offering a vehicle-vehicle communication service and a roadside-vehicle communication service to an on-board unit 102. As illustrated in FIG. 2, a gantry 113 is disposed across a road 111, and roadside communication devices 101-1 to 101-4 are respectively disposed for respective lanes 112 on the gantry 113. An on-board unit 102, which is mounted on a vehicle 103, functions as a mobile station moving with the vehicle 103. The numerals 104-1 to 104-4 respectively denote the communicable regions of the roadside communication devices 101-1 to 101-4 and the numeral 105 denotes the communicable region of the on-board unit 102.
Discussed below is the case when an on-board unit 102 which has been performing a vehicle-vehicle communication with another on-board unit (not illustrated) enters any of the communicable regions of the roadside communication devices 101-1 to 101-4 and it has become a situation to start a roadside-vehicle communication between the on-board unit 102 and the roadside communication devices 101-1 to 101-4. In other words, there arises a necessity of switching the on-board unit 102 from the state of performing the vehicle-vehicle communication to the state of performing the roadside-vehicle communication. Such situation may occur, for example, when the roadside communication devices 101-1 to 101-4 are used in an electronic toll collection (ETC) system and it is necessary to communicate with the roadside communication devices 101-1 to 101-4 for toll charging.
In this case, radio-wave interference may occur, since the communicable regions 104-1 to 104-4 of the roadside communication devices 101-1 to 101-4 overlap the communicable region 105 of the on-board unit 102. In FIG. 2, the region in which radio-wave interference may occur is denoted by the numeral 106.
FIG. 3 is a conceptual illustration illustrating an example in which a communication in a control channel results in failure due to radio-wave interference and switching from the vehicle-vehicle communication to the roadside-vehicle communication thereby results in failure. In the operation illustrated in FIG. 3, control channels (C-CHs) of the same frequency are used for both of the vehicle-vehicle communication and the roadside-vehicle communication. It should be noted that, in FIG. 3, service channel #1 (S-CH#1) is used for the roadside-vehicle communication (communications between the on-board unit 102 and the roadside communication devices 101-1 to 101-4). Service channel #2 (S-CH#2) is, on the other hand, used for the vehicle-vehicle communication (communications between the on-board unit 102 and another on-board unit (not illustrated)).
The roadside communication devices 101-2 and 101-3 try to transmit data to request communications using the service channel #1 to the on-board unit 102 through broadcasting in a control channel. In the meantime, the on-board unit 102 may try to transmit data for the vehicle-vehicle communication to another on-board unit through broadcasting in a control channel of the same frequency. In this case, the on-board unit 102 may fail in reception of data transmitted from the roadside communication devices 101-2 and 101-3 in a control channel due to radio-frequency interference. This may result in that service channels #2 (S-CH#2) are continuously used by the on-board unit 102 and the service channels used by the on-board unit 102 are not switched to service channels #1, that is, switching from the vehicle-vehicle communication to the roadside-vehicle communication results in failure.
From the background described above, there is a need for providing a technology for performing switching between multiple services with radio-wave interference suppressed.
It should be noted that patent literature 2 (Japanese patent application publication No. 2000-165314 A) discloses a technique, which may relate to the present invention, for providing a vehicle-vehicle communication scheme for performing communications between mobile stations without causing disturbance on a roadside-vehicle communication.